1. Field of Invention
The present invention relates generally to laser ablation of conductive films and, more specifically, to a method for laser patterning a multilayered conductor/plastic substrate structure and to multilayered electrode/plastic substrate structures and display devices incorporating the same.
2. Description of the Related Art
A liquid crystal display (LCD) is a type of flat panel display used in various electronic devices. Generally, LCDs comprise two sheets of polarizing material with a liquid crystal solution therebetween. Each sheet of polarizing material typically comprises a substrate of glass or transparent plastic; the liquid crystal (LC) is used as optical switches. The substrates are usually manufactured with transparent electrodes, typically made of indium tin oxide (ITO), to which electrical xe2x80x9cdrivingxe2x80x9d signals are coupled. The driving signals induce an electric field which can cause a phase change or state change in the LC material; the LC exhibiting different light-reflecting characteristics according to its phase and/or state.
Liquid crystals may be nematic, smectic or cholesteric depending upon the arrangement of the molecules. A twisted nematic cell is made up of: two bounding plates (usually glass slides), each with a transparent conductive coating (such as ITO) that acts as an electrode, spacers to control the cell gap, two crossed polarizers (the polarizer and the analyzer), and nematic liquid crystal material. Twisted nematic displays rotate the director of the liquid crystal by 90xc2x0. Super-twisted nematic displays employ up to a 270xc2x0 rotation. This extra rotation gives the crystal a much steeper voltage-brightness response curve and also widens the angle at which the display can be viewed before losing much contrast. Cholesteric liquid crystal (CLC) displays are normally reflective (meaning no backlight is needed) and can function without the use of polarizing films or a color filter. xe2x80x9cCholestericxe2x80x9d means a type of liquid crystal having finer pitch than that of twisted nematic and super twisted nematic. Sometimes it is called xe2x80x9cchiral nematicxe2x80x9d because cholesteric liquid crystal is normally obtained by adding chiral agents to host nematic liquid crystals. Cholesteric liquid crystals may be used to provide bi-stable and multi-stable displays that, due to their non-volatile xe2x80x9cmemoryxe2x80x9d characteristic, do not require a continuous driving circuit to maintain a display image, thereby significantly reducing power consumption. Ferroelectric liquid crystals (FLCs) use liquid crystal substances that have chiral molecules in a smectic C type of arrangement because the spiral nature of these molecules allows the microsecond switching response time that make FLCs particularly suited to advanced displays. Surface-stabilized ferroelectric liquid crystals (SSFLCs) apply controlled pressure through the use of a glass plate, suppressing the spiral of the molecules to make the switching even more rapid.
Some known LCD devices include chemically-etched, transparent, conductive layers overlying a glass substrate. See, e.g., U.S. Pat. No. 5,667,853 to Fukuyoshi et al., incorporated herein by reference. Unfortunately, chemical etching processes are often difficult to control especially for plastic films. As a consequence, electrodes resulting from such processes are often misshaped, with xe2x80x9cwellsxe2x80x9d being formed near the substrate in instances where too much etchant was employed. Moreover, the minimum line gaps obtained in plastic films are typically limited to 15 xcexcm or more. Additionally, concerns for the environment lessen the desirability of employing chemical etching processes which produce dangerous and/or harmful byproducts.
There are alternative display technologies to LCD""s that may be used for example in flat panel displays. A notable example is organic light emitting devices (OLEDs), which are comprised of several layers in which one of the layers is comprised of an organic material that can be made to electroluminesce by applying a voltage across the device. An OLED device is typically a laminate formed on a substrate such as glass. A light-emitting layer of a luminescent organic solid, as well as adjacent semiconductor layers, are sandwiched between an anode and a cathode. The semiconductor layers may be hole-injecting and electron-injecting layers. The light-emitting layers may be selected from any of a multitude of light emitting organic solids, e.g. polymers. When a potential difference is applied across the cathode and anode, electrodes from the electrode-injecting layer and holes from the hole-injecting layer are injected into the light-emitting layer. They recombine, emitting light.
In a typical matrix-addressed light-emitting display device, numerous light emitting devices are formed on a single substrate and arranged in groups in a regular grid pattern. Activation may be by rows and columns, or in an active matrix with individual cathode and anode pads. OLED""s are often manufactured by first depositing a transparent electrode on the substrate, and patterning the same into electrode portions. The organic layer(s) is then deposited over the transparent electrodes. A metallic electrode may be formed over the electrode layers. For example, in U.S. Pat. No. 5,703,436 to Forrest et al., incorporated herein by reference, transparent indium tin oxide (ITO) is used as the hole-injecting electrode, and a Mg-Ag-ITO electrode layer is used for electron injection.
An excimer laser has been employed to pattern ITO electrode material overlying a glass or quartz substrate. See, e.g., U.S. Pat. No. 4,970,366 to Imatou et al. and European Patent Specification EP 0 699 375 B1 by Philips Electronics N.V., both incorporated herein by reference. However, electrode/substrate structures formed with glass or quartz substrates lack the flexibility and thickness desired for many display products.
F. E. Doany et al., xe2x80x9cLarge-field scanning laser ablation systemxe2x80x9d, IBM Journal of Research and Development, Vol. 41, No. xc2xd, 1997, incorporated herein by reference, discloses a large-field scanning laser ablation system which employs a XeCl 308 nm excimer laser and a mask for ablating vias (down to 8 xcexcm) in a polyimide layer. The system employs a projection lens (Dyson-type) to image a portion of a full-field mask onto a portion of the substrate. The system also includes a light tunnel/homogenizer which outputs a square beam with uniformity of xc2x15% across the entire output field, producing an 8-mmxc3x978-mm illumination spot at approximately 0.05 NA.
Excimer lasers have also been used to manufacture thin-film transistors (TFTs). For example, xe2x80x9cFlat-Panel Displays Slim Down with Plasticxe2x80x9d, Science and Technology Review, Nov. 1999, incorporated herein by reference, discloses using an excimer laser to manufacture TFTs on top of thin, plastic sheets. In this reference, an amorphous silicon dioxide layer acts as a thermal barrier to prevent the plastic (PET) substrate from heating and melting. See also, U.S. Pat. No. 5,714,404 to Mitlitsky et al. and U.S. Pat. Nos. 5,817,550 and 5,856,858 to Carey et al., both incorporated herein by reference, which disclose using an excimer laser for crystallizing a TFT silicon layer and for doping.
It is also known to employ an infra-red (IR) fiber laser for patterning a metallic conductive layer overlying a plastic film, directly ablating the conductive layer by scanning a pattern over the conductor/film structure. See: Int. Publ. No. WO 99/36261 by Polaroid Corporation; and Chu et al., xe2x80x9c42.2: A New Conductor Structure for Plastic LCD Applications Utilizing xe2x80x98All Dryxe2x80x99 Digital Laser Patterning,xe2x80x9d 1998 SID International Symposium Digest of Technical Papers, Anaheim, Calif., May 17-22, 1998, no. VOL. 29, May 17, 1998, pages 1099-1101, both incorporated herein by reference. However, metallic conductive layers formed from silver-based, transparent, conductor materials are relatively expensive. Moreover, employing the aforementioned direct lasering techniques is relatively slow and requires complex laser control mechanisms and algorithms to control and direct the narrow IR laser beam.
Accordingly, a high-speed, high-precision, chemical-free method for patterning conductor/plastic substrate structures is needed. To this end, it would also be desirable to have available a flexible conductor/plastic substrate structure with a xe2x80x9cglass replacementxe2x80x9d structure which includes material insulating the glass replacement structure from heat generated during laser irradiation of the conductor/substrate structure. A method for patterning conductor/substrate structures which is sufficiently fast to accommodate a roll-to-roll manufacturing process xe2x80x9cdownstreamxe2x80x9d of the patterning process would also be useful and potentially yield cost savings in the manufacturing of LCD devices.
The present invention is embodied in laser-etched multilayered electrode/substrate structures and methods for manufacturing the same. In a preferred embodiment, a projection-type excimer laser system is employed to rapidly and precisely ablate a pattern from a mask into at least one conductive layer of a multilayered conductor/plastic substrate structure.
In a preferred embodiment, the multilayered conductor/plastic substrate structure includes a xe2x80x9cprotective layerxe2x80x9d which creates a xe2x80x9ccontrolled environmentxe2x80x9d for the laser etching process. The protective layer (e.g., hard coat) serves to protect layers beneath the protective layer from damage caused by laser irradiation of the multilayered electrode/plastic substrate structure. This layer facilitates and speeds the laser etching process. Advantageously, this xe2x80x9cprotective layerxe2x80x9d is a functional layer of a xe2x80x9cglass replacementxe2x80x9d composite, as discussed below.
In a preferred embodiment, the multilayered conductor/plastic substrate structure incorporates one or more functional layers therein. The one or more functional layers of multilayered electrode/plastic substrate structure serve to insulate, promote adhesion, protect layers underneath from laser irradiation, provide protection from environmental damage, and/or provide protection from structural damage, for example, scratches or cracks in the film.
In a preferred embodiment, the at least one functional layer includes a xe2x80x9cbarrier layerxe2x80x9d (e.g., SiOx) which provides xe2x80x9cenvironmental protectionxe2x80x9d for the plastic substrate. xe2x80x9cEnvironmental protectionxe2x80x9d means serving to provide barrier properties against oxygen and/or moisture.
The plastic substrate as constructed with the one or more functional layers can be seen as a xe2x80x9cglass replacementxe2x80x9d structure, in that various properties of the structure are intended to duplicate various characteristics of the glass substrate, such as the aforementioned barrier properties. The glass replacement structure can be a composite of these layers (xe2x80x9cglass replacementxe2x80x9d composite), or a single layer where the functional properties are incorporated through, for example, compounding or coextrusion of the plastic substrate (xe2x80x9cglass replacementxe2x80x9d layer).
A multilayered electrode/substrate structure in accordance with one embodiment of the present invention includes: a plastic substrate; and at least one conductive layer overlying the plastic substrate, the at least one conductive layer being excimer laser-etched into a plurality of discrete conductive elements. In a preferred embodiment, the at least one conductive layer includes an ITO layer which is polycrystalline. In a preferred embodiment, the at least one functional layer serves to: electrically insulate the discrete conductive elements; promote adhesion of the at least one conductive layer to the plastic substrate; protect the plastic substrate from laser irradiation; protect one or more other functional layers including a barrier layer from laser irradiation; protect the plastic substrate from environmental damage caused by exposure to oxygen and/or moisture; or a combination of the above.
A multilayered electrode/substrate structure in accordance with another embodiment of the present invention includes: a plastic substrate; at least one conductive layer overlying the plastic substrate; and at least one functional layer intermediate the plastic substrate and the at least one conductive layer, the at least one functional layer including an insulating material; wherein portions of the at least one conductive layer are excimer laser etched. In a preferred embodiment, the at least one conductive layer includes an ITO layer which is polycrystalline. In a preferred embodiment, the at least one functional layer includes a protective layer which serves to protect layers beneath the protective layer from laser irradiation. In a preferred embodiment, portions of the protective layer underlying the etched portions of the at least one conductive layer are not completely decomposed. In a preferred embodiment, the at least one functional layer includes one or more barrier layer which serves to protect the plastic substrate from environmental damage. In a preferred embodiment, the multilayered electrode/substrate structure further includes an additional functional layer abutting a side of the plastic substrate that faces away from the at least one conductive layer, the additional functional layer serving to provide structural protection and/or environmental protection for the plastic substrate.
A multilayered electrode/substrate structure in accordance with another embodiment of the present invention includes: a substrate; a layer of indium tin oxide (ITO) which is polycrystalline; and at least one functional layer, at least one of which serves as an adhesion promoter of the ITO layer to the substrate; wherein portions of the ITO layer are excimer laser etched. In a preferred embodiment, the at least one functional layer includes a protective layer which serves to protect layers beneath the protective layer from laser irradiation. In a preferred embodiment, portions of the protective layer underlying the etched portions of the at least one conductive layer are not completely decomposed. In a preferred embodiment, the at least one functional layer includes a barrier layer which serves to protect the plastic substrate from environmental damage. In a preferred embodiment, the multilayered electrode/substrate structure further includes an additional functional layer abutting a side of the plastic substrate that faces away from the at least one conductive layer, the additional functional layer serving to provide structural protection and/or environmental protection for the plastic substrate.
A liquid crystal display device in accordance with another embodiment of the present invention incorporates any of the multilayered electrode/substrate structures described herein.
A method for patterning a multilayered conductor/substrate structure in accordance with another embodiment of the present invention includes the steps of: providing a multilayered conductor/substrate structure which includes a plastic substrate and at least one conductive layer overlying the plastic substrate; and irradiating the multilayered conductor/substrate structure with ultraviolet radiation such that portions of the at least one conductive layer are removed therefrom such as through ablation. Preferably, the ultraviolet radiation is spatially incoherent. According to a preferred method, the irradiating step includes employing an excimer laser to ablate portions of the at least one conductive layer. Preferably, the excimer laser is part of a projection-type ablation system which is configured to project a broadened laser beam. Preferably, the excimer laser is controlled in consideration of how well the at least one conductive layer absorbs radiation at particular wavelengths. Preferably, the excimer laser is controlled to image a pattern from a mask onto the at least one conductive layer. Preferably, a fluence of the excimer laser is controlled in consideration of an ablation threshold level of the at least one conductive layer. According to a preferred method, the excimer laser is employed and controlled to ablate portions of the at least one conductive layer without completely decomposing the layer therebeneath. In a preferred embodiment, the layer therebeneath is one or more functional layers.
The above described and many other features and attendant advantages of the present invention will become apparent as the invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings.